The IP Multimedia Subsystem (IMS) is an Internet protocol multimedia subsystem in the 3rd Generation Partnership Project (3GPP) network, and will become a target application network for the Long Term Evolution (LTE) in implementing packet voice and Multimedia Messaging Service.
The IMS adopts an Internet Protocol (IP) packet switched domain as a bearer channel of control signalling and media transmission for the IMS, and adopts the SIP as a call control signalling so as to realize separation of the three operations of service management, session control, and bearer access.
The IMS is a standard under ongoing improvement, and has not achieved complete interconnection with the traditional circuit switched domain network as long as the LTE network is concerned. The interconnection of services of the LTE network and the traditional circuit switched domain network is especially important since the packet network and the traditional circuit switched domain network coexist in a very long period of time. How to solve the problem of interconnection between the IMS network and the traditional circuit switched domain during a call process will be a problem that needs to be solved by the LTE network.
At present, there have been relevant patents and standards dealing with the problem in this field, wherein the focus is on call efficiency and processing of signal tone of another party. Current relevant technical solutions mainly include:
Existing Technical Solution 1:
FIG. 1 illustrates a process in a Public Switched Telephone Network (PSTN) in the current 3GPP standard. It can be seen from FIG. 1 that, before Initial Address Message (IAM) addressing is performed, a media gateway control unit has performed a series of H.248 protocol interactions. The media gateway control unit does not control to open a media stream channel through the H.248 protocol until a traditional circuit switched domain network sends an Answer Message (ANM). If a called User Equipment (UE) is not found during the IAM addressing, the series of processes performed before the IAM addressing are waste of resources. In addition, the ringtone and prompt tone of the called party cannot be received since the media stream channel is opened after the ANM is sent.
Existing Technical Solution 2:
An “early media” technique is provided in RFC (Request for Comments) 3959 released by the Internet Engineering Task Force (IETF) to open the media stream channel after a media negotiation in an Offer/Answer mode is completed by a calling user and a called user. Although this method can solve the above problem, a new problem it brings about is that: in the above requirement, the “early media” technique cannot be directly used between the calling user and the called user when the two belong to different networks respectively, and the media negotiation is performed between the calling UE and a media gateway control unit. Although this solution can solve the problem of receiving the ringtone and prompt tone of the called user during the call, there also exists a greater consumption of resources at the same time, since the media gateway control unit initiates a call to a PSTN after the media negotiation is completed. All the processes performed before the IAM addressing are waste of resources if the IAM addressing fails.
Existing Technical Solution 3:
As shown in FIG. 2, when receiving an Address Complete Message (ACM) from a traditional circuit switched domain network, a media gateway control unit instructs a media gateway to open a media stream channel. When a media control equipment of a calling UE receives an 180Ring (ringing) response message, the media stream channel is directly opened if a media negotiation in the Offer/Answer mode has been completed; otherwise, a calling UE begins the media negotiation in the Offer/Answer mode with the media gateway control unit, and the media stream channel is opened after the media negotiation is completed. Based on this process, another solution corresponding to the process is shown as in FIG. 3, where after receiving an Invite (invite request) message, the media gateway control unit will not perform the media negotiation process in the Offer/Answer mode with the calling UE immediately, but begins to do so through the 180Ring or an 183 session progress response message after receiving the ACM from the traditional circuit switched domain network. After the media negotiation is completed, the media control equipment of the calling user opens the media channel.
However, the technical solutions above fail to take into consideration the following points:
(1) since cross network addressing is performed, the process of the addressing generally takes a long period of time, during which there is no status indication for the calling user, leading to poor user-friendliness;
(2) during the IAM addressing, a lot of time will be spent in vain waiting forever for the traditional circuit switched domain network to feed back an IAM addressing fail message when a problem occurs to the traditional circuit switched domain network or the called UE is out of a service area; and
(3) when media negotiation in the Offer/Answer mode is performed before the IAM addressing, the previous invalid media negotiation in the Offer/Answer mode would be an extra waste of resources in case the address of the called user cannot be found.